This Let_syntax allows you to write expressions like
let open Incr.Let_syntax in
let%map_open some_incr = watch some_variable
and another_incr = ...
and ...
in
...expression involving some_incr, another_incr, etc...
Note that this is less efficient than using map3, map4, etc., as the latter
produces fewer intermediate nodes.
val return : 'a ‑> 'a tmodule Let_syntax : sig ... end